The present invention relates to embodiments of an AC magnetic tracker with frequency-based compensation for distortion caused by metals.
As is well known, eddy currents are induced in conductive metals through the presence of changing magnetic fields. This is a particularly serious problem where AC current is used since where the AC current is operating at any frequency, eddy currents are induced in nearby conductive metals throughout each measurement period.
In systems using magnetic fields to measure position and orientation of objects in a prescribed space in six degrees of freedom, some have attempted to solve some of the problems inherent in AC-based devices by devising systems that use direct current (DC). In such systems, eddy currents are still induced by the rising and falling edges of the DC magnetic pulse. However, delaying of field measurements until sometime after the rising edge allows eddy currents to decay significantly and for measurements to be taken that are substantially free of eddy current distortions. However, use of DC-based systems increases the length of time expended in conducting each cycle of measurements.
Systems have been devised in which the environment in and adjacent the measuring space is xe2x80x9cmappedxe2x80x9d so that the locations of metallic objects are known as is their effect on magnetic measurements. A look-up table is provided to store these measurements so that compensation may be made for prospective eddy current distortion. Such a method is quite tedious and is only effective if the environment in and around the measuring space stays the same at all times, that is, no metallic object or objects are moved from their original locations of placement and no such objects are moved adjacent the measuring space at any other time. In addition, such measurements depend upon a particular defined location for the transmitter coils. Movement of any objects and/or the transmitter coils requires re-measuring and mapping of the environment.
Ferrous objects (with a relative permeability xcexcr greater than 1), when placed in a magnetic field, acquire magnetization that is referred to as induced magnetization. The magnitude and direction of the induced magnetization is a function of the primary field strength, ferromagnetic susceptibility of the body, its shape, and its orientation with respect to the primary field. Ferrous objects may also have permanent or intrinsic magnetization, usually called remnant magnetization. Remnant magnetization is a function of metallurgical properties of the object as well as its thermal, mechanical, and geomagnetic history. Remnant magnetization can be very difficult to model since its strength and direction are often unknown for an individual object. No prior art system teaches compensation for distortion caused by ferrous metals.
As such, a need has developed for a position measuring system that can accurately measure the position and orientation of objects in a measuring space in the six degrees of freedom x, y, z, azimuth, elevation and roll, and which can do so regardless of the particular locations of metallic objects in and adjacent the measuring space. A need has also developed for such a system which can operate quickly without the need to wait for eddy current distortion to decay and wherein the system is less sensitive to ferrous metals, magnetic materials, dynamic filtering and environmental noise.
The following prior art is known to Applicant:
U.S. Pat. Nos. 4,346,384 and 4,328,548 and 4,314,251 and 4,298,874 and 4,054,881 and 4,017,858 and 3,983,474 and 3,868,565 all teach the use of AC magnetic fields in order to measure the position and orientation of a sensor. These patents teach use of different algorithms including 1) Nutating vectors, 2) Far-field, 3) Near-field, 4) Iterative solutions, and 4) Direct solutions.
Some of the patents teach the use of mapping the environment, that is, before the position and orientation measurements are made, storing the mapping data in memory and using look-up tables in order to compensate for eddy current distortion caused by metals in the environment during position and orientation measurements. No direct compensation for the metallic distortions, during position and orientation measurements, are made.
U.S. Pat. Nos. 4,287,809 and 4,394,831 teach position and orientation systems utilizing AC magnetic fields. They utilize non-coplanar transmitter coils and non-coplanar receiver coils. They teach mapping of the environment before the position and orientation measurements are made, storing the mapping data in memory and the use of look-up tables in order to compensate for eddy current distortion for metals in the environment during position and orientation measurements. No direct compensation for the metallic distortions is made during position and orientation measurements.
U.S. Pat. No. 4,829,250 teaches the use of multifrequency transmitters in a position and orientation system. The system utilizes AC magnetic fields and curvefitting is used in order to compensate for the eddy current distortion in conductive metals. The eddy current distortion of conductive metals varies as a function of the frequency and the conductivity. The eddy current distortion decreases with lower frequencies and is zero for DC fields. This method works well if only one conductive metal is present in the work area. If multiple conductive metals are present in the work area, problems may arise. If ferrous metals are present in the work area, a serious problem arises, since the magnetic distortion from ferrous metals is a function of the permeability and the frequency. This distortion decreases with higher frequencies directly opposite the eddy current distortion for conductive metals. The metallic distortion for ferrous metals is not equal to zero at DC. This means that the system will not compensate for metallic distortion if ferrous metals are present. No compensation is made for ferrous metallic distortion.
U.S. Pat. No. 5,347,289 teaches the use of a rotating multifrequency AC magnetic field in a position and orientation system. The system utilizes a special timing sequence of the rotating field in order to compensate for the eddy current distortion from conductive metals. The eddy current distortion decreases with lower frequencies and is zero for DC fields. This method works well if only one conductive metal is present in the work area. If multiple conductive metals are present in the work area, problems may arise. If ferrous metals are present in the work area, a serious problem arises, since the magnetic distortion from ferrous metals is a function of the permeability and the frequency. This distortion decreases with higher frequencies directly opposite with respect to the eddy current distortion for conductive metals. The metallic distortion for ferrous metals is not equal to zero at DC. This means that the system will not compensate for metallic distortion if ferrous metals are present. No compensation is made for ferrous metallic distortion.
U.S. Pat. Nos. 5,694,041 and 5,457,641 teach the use of AC magnetic fields in a position and orientation system. These patents teach the need for an initial mapping of the environment and storing of this data into memory. The data is then used to correct the position and orientation measured during operation. No direct compensation is made for the conductive and ferrous metal distortion.
U.S. Pat. Nos. 5,646,524 and 5,646,525 teach the use of AC magnetic fields in a position and orientation system. They utilize a rotating magnetic field and teach the use of mapping the environment before the position and orientation measurements are made, storing the mapping data in memory, and the use of look-up tables in order to compensate for eddy current distortion for metals in the environment during position and orientation measurements. No direct compensation of the metallic distortions is made during position and orientation measurements.
In U.S. Pat. Nos. 4,849,692 and 4,945,305, a remote object""s position and orientation are determined. These systems utilize a plurality of pulsed DC magnetic fields with steady state components. The steady state components of the generated magnetic fields are measured by DC-field sensitive sensors. These systems reduce the field distortions resulting from the decay of the eddy currents induced in electrically conductive materials by magnetic fields. These systems wait a long enough time for the eddy currents to decay to a sufficiently small value before measuring the steady state component of the generated magnetic fields. These patents teach the need for generating magnetic fields with a steady state component. They further teach the need for using a complex, bulky and expensive active DC-field sensitive sensor. They yet further teach the need for compensation for the Earth""s magnetic field by measuring it when no magnetic field is being generated. This is not an AC magnetic field system. The patents compensate for some conductive metal distortions. Problems do exist with the Earth""s magnetic field, ferrous metallic distortions and environmental noise.
In U.S. Pat. No. 5,453,686, a remote object""s position and orientation are determined. The system utilizes a plurality of pulsed DC magnetic fields with steady state components. The steady state components of the generated magnetic fields are measured by passive sensors. The system reduces the field distortions resulting from the decay of eddy currents induced in electrically conductive materials by magnetic fields. The system waits enough time for the eddy currents to decay before measuring the steady state component of the generated magnetic fields. This patent teaches the need for generating pulsed DC magnetic fields with a steady state component, and the need for using a single coil with an integrator and a reset switch in order to sense the generated pulsed DC magnetic fields. This patent yet further teaches the need for compensation for movements in the Earth""s magnetic field and offsets in the preamplifiers and integrators by measuring the sensors"" output while no magnetic field is being generated. This is not an AC magnetic field system, and does compensate for some conductive metal distortions. Problems with ferrous metallic distortions and environmental noise are not solved.
In U.S. Pat. No. 5,767,669, magnetic pulses are used in order to compensate for the eddy current distortion for a magnetic position and orientation system. Some embodiments teach the use of pulses with a steady state component and some embodiments teach the use of pulses without any steady state components. The systems utilize passive sensors and are an improvement on the pulsed DC systems taught in U.S. Pat. Nos. 4,849,692; 4,945,305; and 5,453,686, but one still needs to wait at least one metal decay time in each pulse in order to compensate for the conductive metal distortion. This is not an AC magnetic field system and, although the systems compensate for some conductive metal distortions, problems do exist with ferrous metallic distortions and environmental noise.
The use of DC pulses severely limits the dynamic performance of the system disclosed in U.S. Pat. No. 5,767,669 and makes it difficult to operate inside a building due to poor filtering of 50/60 Hz current and monitor noise. The DC magnetic systems have serious problems with magnetized ferrous metals (magnetic materials) in the work area due to the measurements of DC magnetic fields and the Earth""s magnetic field. Problems exist in DC magnetic field systems with conductive metals with longer decay times since the pulse must be longer in order to accommodate the longer decay time. This causes more time skew in the measurements, more problems with filtering, and very slow measurement rates, and therefore poor dynamic performance. No prior art DC systems teach any compensation method for ferrous metal distortion as taught herein.
The present invention relates to embodiments of AC magnetic trackers with frequency-based compensation for distortion caused by metals. The present invention is disclosed in eight embodiments. Each of the embodiments utilizes AC magnetic fields to determine position and orientation in the six degrees of freedom (x, y, z) and azimuth (az), elevation (el) and roll (rl). Each of these embodiments utilizes the theory, knowledge of metals, and calculations developed for magnetic geophysical prospecting such as described in Grant et al. xe2x80x9cInterpretation of Applied Geophysicsxe2x80x9d, New York McGraw-Hill 1965, and for metal detectors for the selection of transmission frequencies for the magnetic fields and methods of compensation. The eight embodiments are briefly described as follows:
(1) The first embodiment of the present invention includes at least one AC magnetic transmitter and at least one magnetic receiver. The system transmits at a frequency, f1, low enough for the conductive metals with the highest conductivity to have an in-phase signal approximately equal to zero. The system is capable of measuring the received signal in-phase with the transmitted signal and stores the received signal generated at f1, this signal being free of any eddy current distortion and therefore free of any conductive metal distortion. The system calculates the position (x, y, z) and the orientation (az, el, rl) of the receiver relative to the transmitter based on signals free from distortion from metals.
(2) The second embodiment includes at least one AC magnetic transmitter and at least one magnetic receiver. The system transmits at least two frequencies, f11, and f12, low enough so that no metals have reached their inductive limit. The system is capable of measuring the received signal for each of these frequencies in-phase and in-quadrature to the transmitted signal, I11, Q11 and I12, Q12. The system calculates the metal distortion free signal from the four received signals, and calculates the position (x, y, z) and the orientation (az, el, rl) of the receiver relative to the transmitter based on signals free from distortion from metals.
(3) The third embodiment of the present invention includes at least one AC magnetic transmitter and at least one magnetic receiver with the system transmitting at a frequency, f0, at which distortion from ferrous metals is zero for the in-phase signal. This frequency is found from a frequency graph of ferrous metals and/or knowledge as to the size, shape, conductivity and permeability of the metal. The system is capable of measuring the received signal in-phase with the transmitted signal with the received signal generated at f0, being free of any ferrous metal distortion. The system calculates the position (x, y, z) and the orientation (az, el, rl) of the receiver relative to the transmitter based upon signals free from distortion from ferrous metals.
(4) The fourth embodiment of the present invention includes at least one AC magnetic transmitter and at least one magnetic receiver, with the system transmitting at a frequency, fh, at which distortion from the ferrous metal has reached the inductive limit. The system further transmits at a frequency, f1, where the in-phase signal is approximately equal to zero. The system is capable of measuring the received signal in-phase with the transmitted signal. The signal at fh, free from ferrous metal distortion, is determined from the two in-phase received signals. The system calculates the position (x, y, z) and the orientation (az, el, rl) of the receiver relative to the transmitter based upon signals free from distortion from ferrous metals.
(5) The fifth embodiment of the present invention includes at least one AC magnetic transmitter and at least one magnetic receiver, the system transmitting at a frequency, fh, high enough for both the conductive and the ferrous metals in the environment to have reached their inductive limit. The system stores the received signal generated at fh and then transmits a transition signal in which the transmission transition is made from the maximum level to the zero level in a prescribed short time. The system measures the metal""s inductive limit by measuring the received magnetic field right after the transition. The inductive limit found from the transition signal is either added to or subtracted from the signal received at fh in order to determine the received signal free of metal distortion from conductive as well as ferrous metals. If a second measurement of the received transition signal is higher than the first measurement, the inductive limit is added, and if a second measurement of the received transition signal is smaller than the first measurement, the inductive limit is subtracted. The system calculates the position (x, y, z) and the orientation (az, el, rl) of the receiver relative to the transmitter based upon signals free from distortion from conductive and ferrous metals.
A number of variations to the fifth embodiment are contemplated since it may not be practical to utilize as high a frequency as the needed frequency fh.
(5a) The first variation of the fifth embodiment is to utilize a frequency as practical fhp and calculating the expected quadrature signal if the metal was purely conductive using the decay constant T and the inductive limit found from the transition signal. If the calculated quadrature Qc (if fhp=fT, then Qc=xc2xd inductive limit) is equal to the measured quadrature signal Qhp at fhp, then the metal is purely conductive and the signal free of metal distortion can be found from I=Ihpxe2x88x92+kQc where k is found from the frequency and the decay constant (if fhp=fT, then k=2). If Qc is not equal to Qhp, then the metal is ferrous and the signal free from distortion is found from:
A) Transmitting at a low enough frequency f1 where the in-phase signal of the metal is approximately equal to the value at DC. The signal free of metal distortion can then be found from the signal at f1 and the inductive limit as I=I1+xe2x88x922* (inductive limit); or
B) Transmission of an additional frequency, between fT and fhp, fn, and then using curve fitting on the quadrature signals Qn and Qhp in order to find frequency fh where the Qh=0. It is then possible to find Ih which is the received signal when both the conductive and ferrous metal have reached the inductive limit. The signal free of metal distortion is then found from I=Ih+xe2x88x92inductive limit.
(5b) The second variation of the fifth embodiment is to utilize a frequency f0, which is found from the decay time constant as f0=k0*1/T, where k0 is determined from the permeability of the metal. Then calculating the expected quadrature signal if the metal was purely conductive using the decay constant T and the inductive limit found from the transition signal. If the calculated quadrature Qc signal is equal to the measured quadrature signal Q0 at f0, then the metal is purely conductive and the signal free of metal distortion can be found from I=I0xe2x88x92+kQc where k is found from the frequency and the decay constant. If Qc is not equal to Q0, then the metal is ferrous and the signal free from distortion is I=I0, since f0 is the frequency where the in-phase signal of the ferrous metal is equal to zero.
(6) The sixth embodiment of the present invention includes at least one AC magnetic transmitter and at least one magnetic receiver, the system transmitting at a frequency, fh, high enough for both the conductive and the ferrous metals in the environment to have reached their inductive limit. The system stores the received signal generated at fh, and then transmits a transition signal where the transmission transition is made from the maximum level to the zero level in a predetermined short time. The system measures the metal""s inductive limit by measuring the received magnetic field right after the transition, with the inductive limit found from the received transition signal being either added or subtracted from the signal received at fh in order to determine the received signal free of metal distortion from conductive as well as ferrous metals. If a second measurement of the received transition signal is higher than the first measurement, the inductive limit is added, or if a second measurement of the received transition signal is smaller than the first measurement, the inductive limit is subtracted. The system further includes means to determine a decay time constant of the received transition signal and further means to transmit an AC signal at approximately f=1/T. The system is capable of measuring the received signal in-quadrature to the transmitted signal, QT, and the new inductive limit can be found from this AC signal, thus allowing the system to avoid using the transition signal from time to time. The system calculates the position (x, y, z) and the orientation (az, el, rl) of the receiver relative to the transmitter based on signals free from distortion from conductive and ferrous metals.
(7) The seventh embodiment of the present invention includes at least one AC magnetic transmitter and at least one magnetic receiver. The system transmits at a frequency, fh, high enough for both the conductive and the ferrous metals in the environment to have reached their inductive limit. The system stores the received signal generated at fh, and then transmits a transition signal where the transmission transition is made from the maximum level to the zero level in a predetermined short time. The system measures the metal""s inductive limit by measuring the received magnetic field right after the transition. The inductive limit found from the transition signal is either added or subtracted from the signal received at fh in order to determine the received signal free of metal distortion from conductive as well as ferrous metals. If a second measurement of the received transition signal is higher than the first measurement, the inductive limit is added or if a second measurement of the received transition signal is smaller than the first measurement, the inductive limit is subtracted. The system further includes means for finding the decay time constant of the received transition signal and further means to transmit a second AC signal at approximately f=1/T and a third AC signal at approximately f=k*1/T. The system is capable of measuring the received signal for each of these frequencies in quadrature to the transmitted signal, QT and QkT. The new inductive limit can be found from these two AC signals, thus allowing the system to avoid using the transition signal from time to time. If QT(t)/QkT(t)=QT(0)/QkT(0), then the transition signal is not used, otherwise, the transition signal is used. The system calculates the position (x, y, z) and the orientation (az, el, rl) of the receiver relative to the transmitter based on signals free from distortion from conductive and ferrous metals.
(8) The eighth embodiment of the present invention includes at least one AC magnetic transmitter and at least one magnetic receiver. The system transmits at a frequency, fh, high enough for both the conductive and the ferrous metals in the environment to have reached their inductive limit. The system stores the received signal generated at fh, and further includes means for transmitting a second AC signal at f11, and a third AC signal at f12, both f11 and f12 being lower than fh. The system is capable of measuring the received signal for each of these frequencies in-phase and in-quadrature to the transmitted signal, I11, Q11 and I12, Q12. The system calculates the inductive limit for the metals from the four received signals. The inductive limit is either added to or subtracted from the signal received at fh in order to determine the received signal free of metal distortion from conductive as well as ferrous metals. If I12 greater than I11, the inductive limit is added, and if I12 less than I11, the inductive limit is subtracted. The system calculates the position (x, y, z) and the orientation (az, el, rl) of the receiver relative to the transmitter based upon signals free from distortion from conductive and ferrous metals.
A number of variations to the eighth embodiment are made since it may not be practical to utilize as high a frequency as the needed frequency fh.
(8a) The first variation of the eighth embodiment is to utilize a frequency as practical fhp and calculating the expected quadrature signal if the metal was purely conductive using the decay constant T and the inductive limit found from I11, Q11, I12 and Q12. If the calculated quadrature Qc (if fhp=fT, then Qc=xc2xd inductive limit) is equal to the measured quadrature signal Qhp at fhp, then the metal is purely conductive and the signal free of metal distortion can be found from I=Ihpxe2x88x92+kQc where k is found from the frequency and the decay constant (if fhp=fT, then k=2). If Qc is not equal to Qhp, then the metal is ferrous and the signal free from distortion is found from:
A) Transmitting at a low enough frequency f1 where the in-phase signal of the metal is approximately equal to the value at DC. The signal free of metal distortion can then be found from the signal at f1 and the inductive limit as I=I1+xe2x88x922* (inductive limit); or
B) Transmission of an additional frequency, between fT and fhp, fn, and then using curve fitting on the quadrature signals Qn and Qhp in order to find frequency fh where the Qh=0. It is then possible to find Ih which is the received signal when both the conductive and ferrous metal have reached the inductive limit. The signal free of metal distortion is then found from I=Ih+xe2x88x92inductive limit.
(8b) The second variation of the eighth embodiment is to utilize a frequency f0, which is found from the decay time constant as f0=k0*1/T, where k0 is determined from the permeability of the metal. Then the expected quadrature signal is calculated if the metal was purely conductive using the decay constant T and the inductive limit found from I11, Q11, I12 and Q12. If the calculated quadrature Qc signal is equal to the measured quadrature signal Q0 at f0, then the metal is purely conductive and the signal free of metal distortion can be found from I=I0xe2x88x92+kQc where k is found from the frequency and the decay constant. If Qc is not equal to Q0, then the metal is ferrous and the signal free from distortion is I=I0, since f0 is the frequency where the in-phase signal of the ferrous metal is equal to zero.
Each of the eight embodiments and their variants may be used for any kind of magnetic field position and orientation system, from systems measuring with only one degree of freedom, range r, to a system measuring with six degrees of freedom, position (x, y, z) and orientation (az, el, rl) and any systems measuring with any degrees of freedom between one and six. It can be used in systems using large, flat-in-plane, concentric, non-concentric, co-planar, or non-coplanar transmitter antennas and in systems with any size, concentric, non-concentric, single axis, multi-axis or co-planar or non-coplanar receiving antennas. The system can be used in systems using the relevant formulas for the magnetic field at any distance from the transmitter.
Accordingly, it is a first object of the present invention to provide embodiments of an AC magnetic tracker with frequency-based compensation for eddy current distortion.
It is a further object of the present invention to provide such systems including at least one AC magnetic transmitter and at least one magnetic receiver.
It is a still further object of the present invention to provide such system embodiments in which AC magnetic transmitter means transmits magnetic fields at a frequency low enough for the conductive metals with the highest conductivity to have an in-phase signal approximately equal to zero.
It is a yet further object of the present invention to provide such systems in which two low frequencies, selected so that the metals have not yet reached their inductive limit, are transmitted.
It is a yet further object of the present invention to provide embodiments of an AC magnetic tracker with frequency-based compensation for ferrous metal distortion.
It is a still further object of the present invention to provide such systems in which the AC magnetic transmitter means transmits magnetic fields at a frequency where ferrous metals in the environment in-phase signal are approximately equal to zero.
It is a still further object of the present invention to provide such systems in which the AC magnetic transmitter means transmits magnetic fields at a frequency high enough such that ferrous metals in the environment have reached their inductive limit and at a frequency low enough for the metal""s in-phase signal to be approximately equal to the DC level.
It is a yet further object of the present invention to provide embodiments of an AC magnetic tracker with frequency-based compensation for both conductive and ferrous metal distortion.
It is a still further object of the present invention to provide such systems in which the AC magnetic transmitter means transmits magnetic fields at a frequency high enough such that both conductive and ferrous metals in the environment have reached their inductive limit and a transition signal from which the metal""s inductive limit and decay time constant can be found.
It is a still further object of the present invention to provide such systems in which the AC magnetic transmitter means transmits magnetic fields at a frequency high enough such that both conductive and ferrous metals in the environment have reached their inductive limit and additional means to transmit at two additional frequencies low enough for the metal""s inductive limit to not have been reached, from which the metal""s inductive limit and decay time constant can be found.
It is a still further object of the present invention to provide such systems in which the transition signal does not have to be used from time to time.
It is a still further object of the present invention to provide such systems in which the need for transmission at a frequency high enough for both the conductive and the ferrous metals have reached their inductive limits has been replaced with a need for transmission of a lower frequency.
These and other objects, aspects and features of the present invention will be better understood from the following detailed description of the preferred embodiments when read in conjunction with the appended drawing figures.